The present invention relates to a distance measuring device, especially a device for intermittently transmitting ultrasonic waves or electric waves toward an object, measuring the time required until the transmitted ultrasonic waves or electric waves return to the device after being reflected on the object, and detecting the distance to the object from the measured time.
The present inventors previously have proposed a device for simply and certainly detecting a peak value of even the reflected waves formed like a series of smooth mountains due to damping or the like which occurs while they are being transmitted, and for measuring the distance based on the time required until a peak value of the reflected waves is detected (Japanese Unexamined Patent Publication No. Sho 60-46478). This device enables the measurement of the distance to the object, which is not affected by the hardness of the object, or the temperature and humidity of the transmission medium.
The present inventors have employed the above-described device for detecting the air pressure of tires as shown in FIGS. 5 and 6. In FIGS. 5 and 6, the drop of the air pressure of a tire W is detected by measuring the distance h between an axle A and a road surface E. An ultrasonic distance detector 1 is provided on the axle A and comprises an ultrasonic transmitter and an ultrasonic receiver (not shown), each facing the road surface E. The ultrasonic distance detector 1 further comprises a circuit for detecting a peak value of a reception signal of the ultrasonic receiver and for transmitting a reflected waves reaching signal.
The ultrasonic distance detector 1 also is connected to an operation device 2. The operation device 2 measures the time t needed from the time when the ultrasonic waves are transmitted until the reflected waves reaching signal is received by the ultrasonic distance detector 1, calculates the distance h to the road surface E based on the measured time t and gives an air pressure drop alarm when the distance h becomes smaller than a predetermined value.
However, a large number of experimental results show that the air pressure drop alarm is sometimes erroneously given although the air pressure of the tire is sufficiently high. This is caused by the fact that the time t varies not linearly in proportion to the distance h but periodically at distances of .lambda. as shown by the line y in FIG. 4. This periodical variation of the time t seems to result from the interference between waves reflected on the object. In this case the distance .lambda. is equal to the wave length of waves such as ultrasonic waves used in this device.